The body plan of many animals exhibit bilateral symmetry; this is often reflected in the organization of the nervous system. In both insects and vertebrates the cells which lie along the midline of the nervous system are specified early in development and go on to exhibit distinctive morphologies, patterns of gene expression, and functions. In Drosophila, the embryonic CNS midline is comprised of approximately 25 neurons and glia per segment which play crucial roles int he proper elaboration of the CNS axon scaffold as well as in establishing the identities of neighboring epidermal cells. These cells comprise a discrete, highly accessible cellular subsystem with which to study nerve cell development. Our major interests lie in better understanding the development and function of specific nervous system cell types. Towards this end we will investigate the developmental pathways by which cells of the Drosophila CNS midline differentiate and come to acquire unique functional properties. This will involve molecular genetic analyses of the roles of several genes, including single-minded, other spitz class genes, and a novel midline glial expressed gene, AA41, both in the differentiation of midline neurons and glia and a midline derived signaling pathway required for proper development of the ventral epidermis. These studies will have general implications for understanding the development and function of related midline nervous system structures in other species, such as the vertebrate floor plate and roof plate. In addition, they may shed light onto a series of human genetic and epigenetic birth defects, the holoprosencephalies, which result in midline defects in craniofacial development.